This invention relates to improvements in oil cooled compressors of the kind wherein gas to be compressed, normally the atmospheric air, is drawn together with oil into a compressor member, and the oil acts to cool and lubricate the compressor member. The compressed gas discharged from the compressor member is introduced into an oil tank to separate the oil therefrom and is thereafter directed to a storage tank. The oil separated is cooled and is reused for cooling and lubricating the compressor member. When the pressure in the storage tank elevates to a predetermined maximum value, a pressure switch detects the pressure to open an electric circuit, thereby stopping a motor which drives the compressor. When the air in the storage tank has been consumed and the pressure therein decreases to a predetermined minimum value, the pressure switch closes to start the motor.
The compressed gas discharging out of the compressor member is usually at a high temperature and high humidity, thus, when the oil and the oil tank have not sufficiently been warmed as in the case just after the compressor is started, the compressed gas is cooled in the oil tank, thereby generating drain water which deteriorates the oil and generates rusts on metal parts. Consequently, the oil is preferably at a low temperature in view of the cooling, but it is necessary to maintain the oil at a relatively high temperature to prevent the generation of drain water. This relatively high temperature is determined in accordance with the dew point of the compressed gas and is higher than the room temperature by about 50.degree. C. when the pressure of the compressed air are 8-9 Kg/cm.sup.2. Conventionally, a temperature adjusting valve is interposed between the oil tank and an oil cooler, and the temperature adjusting valve and an oil inlet port of the compressor member is connected by a by-pass tube by-passing the oil cooler, thereby controlling the oil flow in the oil cooler in accordance with the oil temperature.
However, in such prior art devices, it requires a long time to elevate the oil temperature from a relatively low temperature such as the room temperature to a higher temperature, such as the temperature at the dew point of the compressed gas, which can suppress the generation of the drain water. Particularly, when the starting and the stopping of the motor are controlled by a pressure switch detecting the pressure in the storage tank, the operating time of the motor is solely controlled by the pressure and not by the oil temperature. Thus, the oil temperature does not always reach a desired minimum temperature.
It has been proposed to continuously operate the compressor member until the oil temperature elevates to the desired minimum temperature, which can suppress the generation of the drain water, while preventing the pressure of the compressed gas discharged from exceeding a predetermined pressure. Namely, the compressor is operated under an unloading operation, that is, a portion of the compressed gas is released by utilizing a release valve, thereby maintaining the pressure in the storage tank or in the discharge line within a predetermined pressure range. Such a range is called an oil temperature elevating operation range. The energy for operating the compressor in this manner is utilized to heat the oil.
However, in the prior art devices, the desired minimum oil temperature for suppressing the generation of the drain water has been determined without considering the effects of the room temperature. In particular, the desired minimum oil temperature has usually been determined to be 90.degree. C. which corresponds to the expected maximum operating room temperature of 40.degree. C. However, when the room temperature is assumed to be 20.degree. C. the generation of the drain water will not be observed when the oil temperature has been elevated to 20+50=70.degree. C., but the oil temperature elevating operation will be performed until the oil temperature is increased to 90.degree. C. Thus the compressor will more that is required. Further, when the room temperature is sufficiently low, the absolute humidity of the air is very small and essentially no drain water will be generated and, therefore, the oil temperature elevating operation can essentially be omitted.